In the transmission of signals over various transmission media it is generally easier to transmit and process binary signals than analogue signals because of the greater immunity from noise of binary transmission. However, the conventional method of processing the incoming data is to use it to modulate a carrier signal by generating both an in-phase and a quadrature resolved component of the modulating data. Each component is then filtered for bandwidth limiting and pulse shaping. The two filtered baseband signals are then individually mixed with the carrier frequency, the in-phase signal with the carrier itself and the quadrature signal with a 90 degree shifted version of the carrier. The two resulting amplitude modulated signals are then combined and the resulting carrier signal has both phase and amplitude modulation.
There are several disadvantages to the current method. The baseband filtering, quadrature splitting of the carrier and the amplitude balancing of the separate components have to be carefully matched or tuned for matching of the filter characteristics and balancing of the phase and amplitude. Even if such matching and balancing is done the components still produce variations in performance due to temperature variations and aging.